[Show abstract][Hide abstract]ABSTRACT:
We have investigated the possibilities of using Computational Fluid
Dynamics (CFD) simulations to characterize the impact of refractive
index fluctuations in a jet engine plume on Directed InfraRed
CounterMeasure (DIRCM) system performance. The jet plume was modelled
using both Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy
Simulation (LES) formulations of Navier-Stokes equations. The RANS
calculations provided a time-averaged description of the refractive
index and the turbulence strength. The more computationally intense LES
model provided time resolved data on large scale turbulent eddies within
the engine plume. The smaller structures are assumed to be isotropic and
are modelled implicitly to reduce the computational demands to levels
feasible for current computational hardware. The refractive index data
from the CFD calculations was integrated along the optical propagation
path to produce phase screens. For RANS data this approach provided time
averaged aberrations, whereas for LES data the temporal variation of low
spatial frequency aberrations were available for a short time sequence.
Modal descriptions of the phase screens were investigated to allow study
of temporal variation at longer time scales. Alternatively the structure
parameter (Cn2) can be estimated and used to
provide order of magnitude approximations for the optical effects. The
generated phase screens were used to calculate laser beam system level
quality parameters including beam wander, fidelity ratio and
power-in-bucket. The paper focuses on method development, but examples
of a jet plume simulation showing that the engine plume turbulence has a
significant impact on DIRCM system functionality are presented.

[Show abstract][Hide abstract]ABSTRACT:
Snipers and other optically guided weapon systems are serious threats in
military operations. We have studied a SWIR (Short Wave Infrared)
camera-based system with capability to detect and locate snipers both
before and after shot over a large field-of-view. The high frame rate
SWIR-camera allows resolution of the temporal profile of muzzle flashes
which is the infrared signature associated with the ejection of the
bullet from the rifle. The capability to detect and discriminate sniper
muzzle flashes with this system has been verified by FOI in earlier
studies. In this work we have extended the system by adding a laser
channel for optics detection. A laser diode with slit-shaped beam
profile is scanned over the camera field-of-view to detect retro
reflection from optical sights. The optics detection system has been
tested at various distances up to 1.15 km showing the feasibility to
detect rifle scopes in full daylight. The high speed camera gives the
possibility to discriminate false alarms by analyzing the temporal data.
The intensity variation, caused by atmospheric turbulence, enables
discrimination of small sights from larger reflectors due to aperture
averaging, although the targets only cover a single pixel. It is shown
that optics detection can be integrated in combination with muzzle flash
detection by adding a scanning rectangular laser slit. The overall
optics detection capability by continuous surveillance of a relatively
large field-of-view looks promising. This type of multifunctional system
may become an important tool to detect snipers before and after shot.

[Show abstract][Hide abstract]ABSTRACT:
Detection and localisation of optical assemblies used for weapon
guidance or sniper rifle scopes has attracted interest for security and
military applications. Typically a laser system is used to interrogate a
scene of interest and the retro-reflected radiation is detected.
Different system approaches for area coverage can be realised ranging
from flood illumination to step-and-stare or continuous scanning
schemes. Independently of the chosen approach target discrimination is a
crucial issue, particularly if a complex scene such as in an urban
environment and autonomous operation is considered. In this work target
discrimination strategies in optics detection are discussed. Typical
parameters affecting the reflected laser radiation from the target are
the wavelength, polarisation properties, temporal effects and the range
resolution. Knowledge about the target characteristics is important to
predict the target discrimination capability. Two different systems were
used to investigate polarisation properties and range resolution
information from targets including e.g. road signs, optical reflexes,
rifle sights and optical references. The experimental results and
implications on target discrimination will be discussed. If autonomous
operation is required target discrimination becomes critical in order to
reduce the number of false alarms.

[Show abstract][Hide abstract]ABSTRACT:
Detection of optical assemblies is important in revealing threats arising from snipers or other weapons guided by optical
means. Several approaches can be imagined using flood illumination or scanning laser techniques. One challenging
problem in optics detection applications in urban environments, particular if an autonomous approach is chosen, is to
reduce the false alarm rate. In this work a dual channel approach for optics detection using a narrow scanning rectangular
laser beam is described. One channel is used for locating targets in the vertical direction while a second channel
simultaneously determines the distance to the targets. An experimental system consisting of two channels operating at
0.8 micrometer wavelength was used to study the characteristics of different targets such as road signs, optical reflexes,
rifle sights, optical references and backgrounds at different ranges and in different environments. Schemes for refining
the target discrimination, reducing the false alarm rate and improving the performance are discussed using experimental
results. A dual channel approach is suggested to improve capabilities in optics detection using a scanning rectangular
laser beam.

[Show abstract][Hide abstract]ABSTRACT:
Tomographic signal processing is used to transform multiple
one-dimensional range profiles of a target from different angles to a
two-dimensional image of the object. The range profiles are measured by
a time-correlated single-photon counting (TCSPC) laser radar system with
approximately 50 ps range resolution and a field of view that is wide
compared to the measured objects. Measurements were performed in a lab
environment with the targets mounted on a rotation stage. We show
successful reconstruction of 2D-projections along the rotation axis of a
boat model and removal of artefacts using a mask based on the convex
hull. The independence of spatial resolution and the high sensitivity at
a first glance makes this an interesting technology for very long range
identification of passing objects such as high altitude UAVs and
orbiting satellites but also the opposite problem of ship identification
from high altitude platforms. To obtain an image with useful information
measurements from a large angular sector around the object is needed,
which is hard to obtain in practice. Examples of reconstructions using
90 and 150° sectors are given. In addition, the projection of the
final image is along the rotation axis for the measurement and if this
is not aligned with a major axis of the target the image information is
limited. There are also practical problems to solve, for example that
the distance from the sensor to the rotation centre needs to be known
with an accuracy corresponding to the measurement resolution. The
conclusion is that that laser radar tomography is useful only when the
sensor is fixed and the target rotates around its own axis.

[Show abstract][Hide abstract]ABSTRACT:
Photon counting techniques using direct detection has recently gained considerable interest within the laser radar
community. The high sensitivity is of special importance to achieve high area coverage in surveillance and mapping
applications and long range with compact systems for imaging, profiling and ranging. New short pulse lasers including
the super continuum laser is of interest for active spectral imaging. A special technique in photon counting is the "time
correlated single photon counting" (TCSPC). This can be utilized together with short pulse (ps) lasers to achieve very
high range resolution and accuracy (mm level). Low average power lasers in the mW range enables covert operation
with respect to present laser warning technology.
By analyzing the return waveform range and shape information from the target can be extracted. By scanning the beam
high resolution 3D images are obtained. At FOI we have studied the TCSPC with respect to range profiling and imaging.
Limitations due to low SNR and dwell times are studied in conjunction with varying daylight background and
atmospheric turbulence. Examples of measurements will be presented and discussed with respect to some system
applications.

[Show abstract][Hide abstract]ABSTRACT:
A time-correlated single-photon counting (TCSPC) laser radar system can
be used for range profiling of objects with high time resolution and
dynamic range. A system setup is described and daytime outdoor
measurements over distances up to 1 km are presented. The system has 114
ps full width half maximum system response, indicating a Rayleigh
criterion resolution of two surfaces separated by 17 mm and much better
with more advanced signal processing methods. The high dynamic range and
time resolution allows measurement of distances between different
optical surfaces in objects such as optical sights. The system thus has
a potential use to classify objects and remove false alarms in an optics
detection system. Effects of atmospheric turbulence and background
radiation in daytime conditions are analyzed. A method for determining
the scintillation index in noisy data using the temporal autocorrelation
is described. System performance calculations based on measured data
indicate that the performance necessary to detect characteristic
features of optical sights and other retroreflecting objects may be
found in reasonable dwell times well below 100 ms.

[Show abstract][Hide abstract]ABSTRACT:
Atmospheric turbulence effects close to ground may affect the performance of laser based systems severely. The variations in the refractive index along the propagation path cause effects such as beam wander, intensity fluctuations (scintillations) and beam broadening. Typical geometries of interest for optics detection include nearly horizontal propagation paths close to the ground and up to kilometre distance to the target. The scintillations and beam wander affect the performance in terms of detection probability and false alarm rate. Of interest is to study the influence of turbulence in optics detection applications. In a field trial atmospheric turbulence effects along a 1 kilometre horizontal propagation path were studied using a diode laser with a rectangular beam profile operating at 0.8 micrometer wavelength. Single-path beam characteristics were registered and analysed using photodetectors arranged in horizontal and vertical directions. The turbulence strength along the path was determined using a scintillometer and single-point ultrasonic anemometers. Strong scintillation effects were observed as a function of the turbulence strength and amplitude characteristics were fitted to model distributions. In addition to the single-path analysis double-path measurements were carried out on different targets. Experimental results are compared with existing theoretical turbulence laser beam propagation models. The results show that influence from scintillations needs to be considered when predicting performance in optics detection applications.

[Show abstract][Hide abstract]ABSTRACT:
The exhaust from engines introduces zones of extreme turbulence levels in local environments around aircraft. This may disturb the performance of aircraft mounted optical and laser systems. The turbulence distortion will be especially devastating for optical missile warning and laser based DIRCM systems used to protect manoeuvring aircraft against missile attacks, situations where the optical propagation path may come close to the engine exhaust. To study the extent of the turbulence zones caused by the engine exhaust and the strength of the effects on optical propagation through these zones a joint trial between Germany, the Netherlands, Sweden and the United Kingdom was performed using a medium sized military turboprop transport aircraft tethered to the ground at an airfield. This follows on earlier trials performed on a down-scaled jet-engine test rig. Laser beams were propagated along the axis of the aircraft at different distances relative to the engine exhaust and the spatial beam profiles and intensity scintillations were recorded with cameras and photodiodes. A second laser beam path was directed from underneath the loading ramp diagonally past one of the engines. The laser wavelengths used were 1.5 and 3.6 mum. In addition to spatial beam profile distortions temporal effects were investigated. Measurements were performed at different propeller speeds and at different distances from exhaust nozzle to the laser path. Significant increases in laser beam wander and long term beam radius were observed with the engine running. Corresponding increases were also registered in the scintillation index and the temporal fluctuations of the instantaneous power collected by the detector.

[Show abstract][Hide abstract]ABSTRACT:
The exhaust from jet engines introduces extreme turbulence levels in
local environments around aircrafts. This may degrade the performance of
electro-optical missile warning and laser-based DIRCM systems used to
protect aircrafts against heat-seeking missiles. Full scale trials using
real engines are expensive and difficult to perform motivating numerical
simulations of the turbulence properties within the jet engine exhaust.
Large Eddy Simulations (LES) is a computational fluid dynamics method
that can be used to calculate spatial and temporal refractive index
dynamics of the turbulent flow in the engine exhaust. From LES
simulations the instantaneous refractive index in each grid point can be
derived and interpolated to phase screens for numerical laser beam
propagation or used to estimate aberration effects from optical path
differences. The high computation load of LES limits the available data
in terms of the computational volume and number of time steps. In
addition the phase screen method used in laser beam propagation may also
be too slow. For this reason extraction of statistical parameters from
the turbulence field and statistical beam propagation methods are
studied. The temporal variation of the refractive index is used to
define a spatially varying structure constant. Ray-tracing through the
mean refractive index field provides integrated static aberrations and
the path integrated structure constant. These parameters can be used in
classical statistical parameterised models describing propagation
through turbulence. One disadvantage of using the structure constant
description is that the temporal information is lost. Methods for
studying the variation of optical aberrations based on models of Zernike
coefficients are discussed. The results of the propagation calculations
using the different methods are compared to each other and to available
experimental data. Advantages and disadvantages of the different methods
are briefly discussed.

[Show abstract][Hide abstract]ABSTRACT:
Resonant output energy enhancement in a singly resonant nondegenerate type-I optical parametric oscillator with a volume Bragg grating output coupler is demonstrated. The resonances occur when the pump laser and parametric oscillator cavity length ratio is an integer or a fraction of small integers. Although the length resonances are similar to those observed in doubly resonant optical parametric oscillators, the physical mechanism is distinctly different. The resonances in the singly resonant oscillator are caused by correlation of the instantaneous power between the quasi-periodic multimode pump laser beam and the OPO signal.

[Show abstract][Hide abstract]ABSTRACT:
Spectral requirements for the first stage OPO used to pump a tandem ZGP mid-infrared OPO are theoretically investigated. Based on these requirements we demonstrate a singly-resonant type-I OPO including periodically poled KTiOPO(4) and volume-Bragg gratings as output couplers. Singly resonant oscillation is demonstrated very close to degeneracy, where signal and idler spectra are not well separated. Investigations of the longitudinal mode spectrum and the idler spectrum with high resolution using a scanning Fabry-Perot interferometer show the essential role played by the phase correlations of the multi-longitudinal mode Q-switched pump laser in formation of the nonresonant idler spectrum.

[Show abstract][Hide abstract]ABSTRACT:
Optical measurements and tests of optical instruments are often performed through an opened window or from the roof of an elevated building. This can also be a common situation for free-space optical (FSO) communication systems. Wind friction in combination with solar heating of the wall and the ground will create increased turbulence in a boundary layer close to the wall. For an outgoing laser beam this thin region of strong turbulence causes beam wander, beam broadening and beam break-up. For imaging and detection systems angle of arrival fluctuations and image blurring may result. In an attempt to estimate the strength of the atmospheric turbulence in the layer at the wall the refractive index structure constant (Cn2) was measured with an ultra sonic anemometer as a function of distance from the wall. The measurements were performed at the lower part of a window that was open just enough to give space for the anemometer. The window was placed 10 m above ground in a 12 m high building, with brick wall below the window and wooden panel above the window. Measurements of the turbulence as a function of distance from the wall were performed during different times of the day to study the influence of sun heating of the wall. The measured average Cn2 shows an exponentially decreasing function of distance from the wall. The exponential decay of Cn2 depends on the time of the day. The highest measured value of Cn2 was approximately 3x10-11 m-2/3 near the wall. The influence of wall turbulence is discussed with respect to its influence on laser beam propagation.

[Show abstract][Hide abstract]ABSTRACT:
Range profiling with high resolution and accuracy can be accomplished using single-photon counting time-of-flight techniques. Detection of target surfaces with high resolution is of importance for several remote sensing applications. The use of laser pulses in the picosecond regime, single-photon avalanche photodiodes and acquisition electronics with high timing resolution provides the tools for improving the range accuracy. This paper gives examples of measured surface profiles and compares the results with simplified theoretical models. The results are discussed with respect to some applications.

[Show abstract][Hide abstract]ABSTRACT:
Laser beam propagation through adverse turbulent environments such as the region close to a jet engine exhaust need to be studied in order to predict performance degradations on airborne laser systems. The turbulent plume region may introduce severe perturbations which accumulate and cause beam degradation in terms of beam wander, intensity scintillations and beam broadening at longer ranges. Applications of interest with respect to laser beam propagation in jet engine plume environments include e.g. directed infrared countermeasures (DIRCM) and active imaging. By characterising and evaluating the perturbation effects schemes for compensation or avoiding performance degradation can be devised. The turbulence effects in the plume region occur by mixing of hot exhaust flow from the jet engine with surrounding ambient air causing spatial and temporal fluctuations in the refractive index. In comparison to atmospheric turbulence considerably shorter outer- and inner scales have been observed. Typical values of the structure constant within the plume region range from 10-10 to 10-9 m-2/3 making the turbulence several order in magnitude stronger in contrast to propagation through the atmosphere. Of importance in characterisation of the jet engine plume with respect to laser beam propagation are turbulent length scales, the extent of the turbid region, variation of the structure constant and temporal flow properties. In this paper reported experimental results and modelling approaches aimed for predicting laser beam propagation degradation in jet engine plume regions are reviewed. The results will be discussed in perspective of system performance.

[Show abstract][Hide abstract]ABSTRACT:
Airplane based laser systems for DIRCM, active imaging and communication are important applications attracting considerable interest. The performance of these systems in directions where the laser beam points close to or through the exhaust plume from the jet engines may be severely reduced. A trial to study these phenomena using a downscaled jet-engine test rig was carried out. The results on propagation of laser beams along and across the plume from these trials are presented. For laser beams propagation along the engine axis an OPO based source producing co-propagating laser beams at 1.52 and 3.56 mum was used. The beams were projected on a screen and imaged with separate IR cameras to study beam wander and spot degradation. Propagation across the plume was studied with a 532 nm laser projected on a screen and imaged by a high speed camera. The engine thrust and the distance between the engine nozzle and the laser beams were varied to study the effects of changing conditions. Scaling to full size engines and performance implications for DIRCM is discussed.

[Show abstract][Hide abstract]ABSTRACT:
In several laser radar applications detection of targets with high resolution and range accuracy, is of importance. Time-of-flight time-correlated single-photon counting (TCSPC) provides a method to accomplish range profiling at longer ranges with high accuracy and resolution. The performance of a TCSPC system used for optical range profiling suffers from the influence of atmospheric turbulence effects causing perturbations of the registered time histograms. This is mostly manifested in propagation paths close to the ground. In this work a TCSPC system based on a monostatic transmitter/receiver head, a picosecond laser operating at high pulse-repetition frequency, a single photon detector and acquisition electronics with high timing resolution was used to study the influence from atmospheric turbulence on registered pulse responses from test targets. The turbulence conditions were monitored during the experiments and the influence from turbulence effects on the pulse response are discussed. The experimental results are considered in relation to existing turbulence models. Implications on system performance for a TCSPC time-of-flight range profiling system are illustrated.